The demand for smaller thickness and lighter weight is not limited to the housings of mobile electronic devices such as portable computers and cell phones and it has recently become intensive in the field of general electronic devices. In particular, the chassis and the internal mechanistic parts of a copying machine and the like are required to have not only high dimensional precision and various kinds of strength associated with handling but also reduction in thickness and weight. As a result, injection moldings are needed that desirably have unequal section thickness, i.e., the portions that do not require strength are made thin and lightweight whereas the portions that require strength are made thick, and which still have good dimensional precision. In other words, it is required to meet both requirements for strength and lightweight by providing a design in which the portions that require strength are reinforced with thick ribs whereas the portions that do not require strength are made as thin as possible. Under these circumstances, a molding method is needed by which even the sections that are thin-walled and have long flow distances can be adequately filled with a resin during molding.
In order to ensure that even the sections that are thin-walled and have long flow distances can be adequately filled with a resin, one may enhance the flowability of a molten resin. In injection molding of a thermoplastic resin, the flowability of a molten resin determines not only the ease in filling the mold cavity but also the probability that after filling the cavity, sufficient pressure is transmitted to its interior, particularly to the resin which forms the thin-walled portions at the end of resin flowing; hence, the flowability of a molten resin also affects the dimensional precision of moldings and is an important factor that determines the processability of resins.
One index of flowability is the viscosity of a molten resin. Thermoplastic resins have high melt viscosity and are poor in flowability as molding materials. Hence, in the case of thin-walled parts, incomplete resin filling often occurs.
In order to lower the viscosity of a molten resin and thereby improve the flowability, it is effective to increase the molding temperature; however, in the case of a resin for which the molding temperature is close to its decomposition temperature or a resin incorporating special additives such as less heat-stable flame retardants, the resin itself or the additives may undergo thermal decomposition and problems are likely to occur as exemplified by the decrease in the strength of moldings, the formation of foreign matter due to the deteriorated resin, the staining of the mold and discoloration. Yet another problem is delayed cooling of the resin in the mold which contributes to prolonging the molding cycle time.
The following methods are conventionally known to be capable of improving the flowability of molten resin without increasing the molding temperature.
(1) Reducing the molecular weight of the resin by lowering its average molecular weight or broadening the molecular weight distribution, particularly by increasing the low-molecular weight component.
(2) Introducing a comonomer into the molecule.
(3) Adding a low-molecular weight oily substance such as mineral oil or a plasticizer such as a higher aliphatic acid ester.
(4) Dissolving carbon dioxide which acts as a plasticizer.
To further describe the above method (4), as shown in J. Appl. Polym. Sci., Vol. 30, 2633 (1985) and many other references, it is known that carbon dioxide dissolved in a resin works as a plasticizer for the resin to lower its glass transition temperature. In addition, the official gazette of Japanese Patent Laid-Open No. 318541/1993 discloses two methods for producing non-foamed molded articles using thermoplastic resins having their flowability improved with gases; in one method, a thermoplastic resin incorporating a gas such as carbon dioxide in an amount of 10 to 90 vol % as calculated under normal temperature and pressure is injected into a mold cavity that has been forcibly evacuated by, for example, a pump and the mold cavity is held evacuated until dwelling so that molding can be performed with the gas from the thermoplastic resin being forced out of the mold cavity; in the other method, the thermoplastic incorporating the gas is injected into a partially opened mold as accompanied by breathing and then subjected to compression molding.
It is also known to produce foamed molded articles using molten resins having a gas such as carbon dioxide dissolved therein. For example, the specifications of WO 89/00918 and U.S. Pat. No. 5,334,356 disclose methods in which carbon dioxide used as a blowing agent is supplied into a molten resin as it flows part of the way through an extruder, thereby molding a fine and highly foamed microcellular foam.
However, the aforementioned method (1) lowers impact strength and chemical resistance although it increases flowability; the aforementioned method (2) lowers hot rigidity; and the aforementioned method (3) has problems such as the plasticizer lowering hot rigidity or being deposited on the mold to stain it during molding.
The aforementioned method (4) has the advantage of not causing the problems encountered in the above methods (1)-(3); however, since the gas incorporated in the molten resin also functions as a blowing agent, this method suffers the problem that the molded article tends to have a foam structure.
The basic principle of the method described in the official gazette of Japanese Patent Laid-Open No. 318541/1993 is that foaming is suppressed by causing the gas in the molten resin to be rapidly removed before the dwelling step starts. In this method described in the official gazette of Japanese Patent Laid-Open No. 318541/1993, the above-mentioned rapid breathing cannot be effected if more gas is incorporated in the molten resin and this sets considerable limit to the volume of the gas that can be preliminarily incorporated in the molten resin. Specifically, if carbon dioxide is used as gas in the method described in the official gazette of Japanese Patent Laid-Open No. 318541/1993, the amount of carbon dioxide that can be dissolved in the molten resin is as small as about 0.18 wt % at maximum and this is insufficient to achieve the satisfactory improvement in flowability.
Under these circumstances, the present inventors proposed in WO 98/52734 a new method of injection molding which used carbon dioxide as a plasticizer but it still suffered the problem of difficulty in achieving the satisfactory improvement in flowability.
The present invention has been accomplished in the light of the aforementioned problems in the prior art and has as an object ensuring that in injection molding a molten resin having carbon dioxide dissolved therein as a plasticizer, a sufficient amount of carbon dioxide to achieve satisfactory improvement in flowability is dissolved in the molten resin so that a molding can be obtained with significant improvement in the flowability of the molten resin.